Pianos have been built for centuries with soundboard constructed of wood. It is known that a relatively close grained wood is the best for soundboards. It has been conventional in the United States to use Sitka spruce boards of an appropriate thickness and edge-glued together to make up the necessary width of the board.
It is known that the velocity of sound in a solid material is equal to the square root of the elasticity divided by the density. Thus, for best sound propagation the elasticity should be high and the density should be low. Most woods are not suitable for soundboards either because the elasticity is too low or the density is too high. Conventional spruce or pine is not satisfactory since the growth is too fast, and the grain too open. Hence, the density and elasticity are not appropriate. The specific spruce suitable for the construction of solid piano sounding boards is grown under somewhat adverse climatic conditions, whereby the growth is slow, and the grain closely spaced.
Spruce boards for piano soundboards are quarter sawn. Accordingly, a board can be no wider than the radius of a tree. The center and the sapwood must be avoided, whereby the width of such boards is limited. It must also be borne in mind that knots, grain swirls, etc. must be avoided. The preferred grain spacing corresponds to something close to ten annular rings per inch. From this it is easy to see that only very old trees can have any appreciable thickness of boards made from them. As a result of this and other factors the supply of suitable spruce boards is shrinking.
Conventional piano soundboards have an inherent disadvantage in that sound travels fastest in the direction of the grain. In fact, the speed in the grain direction of a conventional solid spruce soundboard is about four times as fast as it is transverse of the grain. Sound waves in a piano soundboard are primarily surface waves in the wood and not compression waves.
Some effort has been made in the past to construct soundboards of laminated plies. Such efforts have not met with success, as the boards have failed to resonate properly, and have sounded dull or "tubby".